Transportation Statistics and Microsimulation

210    ◾    Transportation Statistics and Microsimulation

deviance/DF should be close to 1 if the model ts the data well. A value of

deviance/DF much greater than 1 suggests that the model is misspecied

or there is an overdispersion problem in the data. A value of deviance/

DF much less than 1 also indicates misspecication of the model or an

underdispersion problem (the phenomenon that the variance is less than

the mean, which also violates an underlying assumption for the Poisson

distribution) in the data (Sheather, 2009). Caution needs to be exercised

with using the deviance as a goodness-of-t measure when the data set

being analyzed is ...